1. Field of the Invention
This disclosure relates to a developing unit for stabilizing an amount of a developer inside the unit.
2. Description of the Related Art
An image forming apparatus which has a two-component development type developing unit mounted thereon is widely used. The developing unit develops an electrostatic image (electrostatic latent image) formed on an image carrier into a toner image by using a two-component developer which includes a non-magnetic toner and a magnetic carrier. One type of developing unit has a configuration in which the two-component developer is stirred and circulated by one pair of conveyance screws which is disposed in a longitudinal direction of a developing container while sandwiching a partition.
In the developing unit, a new non-magnetic toner is supplied in accordance with the consumption of the non-magnetic toner. Meanwhile, since the old magnetic carrier continues to circulate in the developing container, the charging performance of the two-component developer gradually deteriorates. For this reason, a so-called trickle configuration, in which the magnetic carrier in the developing container is also replaced, and the charging performance of the magnetic carrier in the developing container is maintained constantly, by discharging the magnetic carrier in the developing container little by little, and replenishing the carrier by mixing a new carrier into a replenishing developer, is adopted for the developing unit.
As the trickle configuration, in JP-A-2002-72686, a developing unit, which supplies the two-component developer for replenishment in which the magnetic carrier is mixed with the non-magnetic toner at a predetermined ratio, and which replaces the magnetic carrier in the developing container little by little along with an image formation, is suggested.
The developing unit described in JP-A-2002-72686 includes a first conveyance screw, and a second conveyance screw which has a conveyance direction opposite to that of the first conveyance screw. The two-component developer which circulates in the developing container is discharged little by little, passing a discharge opening provided on an abutting surface of the conveyance direction of the second conveyance screw. The second conveyance screw includes a main spiral portion which conveys the two-component developer in a circulating direction and feeds the two-component developer into the discharge opening, and a return screw which has a direction of conveyance opposite to that of a main spiral portion is linked to a downstream side of the main spiral portion. As the first and the second conveyance screws rotate in directions opposite to each other, a flow of the developer in the vicinity of a developer discharging portion passes through an opening which is provided on the return screw side in the partition and the developer is delivered to the first conveyance screw from the second conveyance screw.
Since a blade of the first conveyance screw is inclined, a force which conveys the developer in a cross-sectional radial direction of the first conveyance screw is also applied, in addition to the direction of developer conveyance. Furthermore, the developer which is stripped and dropped from a developing sleeve which rotates in the same direction as that of the first conveyance screw, also receives a force in a different direction by centrifugal force. For this reason, the developer which is conveyed in the cross-sectional radial direction of the first conveyance screw and the developer which is stripped and dropped from the developing sleeve collide with a flow of the developer which passes through the opening and is delivered to the first conveyance screw from the second conveyance screw. Therefore, a flow of the developer is formed in a space which is sandwiched between the partition having the opening and the return screw, the developer flows to the outside of the return screw, and the developer falls into the discharge opening and is delivered to a discharge screw. In this manner, since the flow changes due to the rotational speed of the first conveyance screw and the developing sleeve, the flow of the developer in the space between the partition and the return screw largely fluctuates due to the rotational speed of the first conveyance screw and the developing sleeve.
Here, a developing unit having a configuration in which a discharge amount of the developer is restricted by blocking the flow of the developer in the space between the partition and the return screw which largely fluctuates by the rotational speed of the first conveyance screw by a flange member disposed in the space sandwiched between the partition and the return screw, is suggested in JP-A-2010-256701. The flow of the developer which is blocked by the flange member is pushed back by the return screw. In this case, by the flange member, when the first conveyance screw and the developing sleeve rotate at a high speed, the amount of the two-component developer which is discharged through the discharge opening becomes less, and when the first conveyance screw and the developing sleeve rotate slowly, the flow of the two-component developer which passes through the same space toward the discharge opening slows down, and the discharge amount of the two-component developer is small. For this reason, the discharge amount of the two-component developer depends less on the number of rotations of the first conveyance screw and the developing sleeve.
In recent years, it has been required to reduce toner consumption. When toner consumption is reduced, a frequency of replenishment becomes smaller and the amount of replenishing carriers per unit image decreases. In addition, the amount of the replenishing carriers per unit image decreases when images having an extremely low image ratio continues to be generated, when a driving configuration of the developing unit is shared by a plurality of stations, or when a ratio of the carrier which is mixed in the replenishing developer decreases.
However, in the trickle configuration, it is ideal that the amount of the developer is held within a constant range by increasing and decreasing the discharge amount of the developer in accordance with the amount of the developer and by not discharging the developer when the amount of the developer becomes equal to or less than predetermined amount of the developer. However, in the configuration described in JP-A-2010-256701, the flow of the developer in the space between the partition and the return screw decreases by providing the flange member, but when considering component tolerance, a gap between the flange member and the return screw cannot be eliminated. For this reason, even when the amount of developer becomes smaller, the developer is discharged from the gap between the partition and the return screw little by little. Therefore, when the amount of the replenishing carrier per unit image is small, there is a concern of causing a so-called coating defect in which the amount of the developer in the developing container decreases, and the developing sleeve cannot be coated with the developer.